In our studies of the insulin-like growth factor (somatomedin), multiplication stimulating activity (MSA), a polypeptide synthesized by a cloned line of rat liver cells in culture progress has been made in three major areas: 1. Chemical characterization of MSA peptides from BRL-3A rat liver cells; 2. Development of a radioimmunoassay for BRL-MSA; and 3. A demonstration that fetal rat liver explants synthesize a polypeptide related to BRL-MSA.